This invention relates to a video-equipped endoscope that utilizes sequential color imagery to produce a full-color video picture of a remote target and, in particular, to an automatic signal level control for use in a video-equipped endoscope.
With the development of charged couple devices (CCD), it is now possible to equip the insertion tube head of an endoscope with an extremely small video camera that enables the head to be positioned within relatively confined regions that have heretofore been inaccessible to this type of viewing equipment. The video camera generally contains a single black and white CCD image sensor that integrates the light imaged onto its surface while recording a given picture of the target. In order to produce a full-color video picture of the target, the target is illuminated sequentially with light of primary colors to produce color separated images. Typically the primary colors of red, green and blue are used in the video system. The color separated images, after being recorded by the image sensor, are clocked out in the form of video signals to a downstream processor that places the color information in a format which is compatible with most video systems.
In U.S. Pat. No. 4,074,306 to Kakinuma et al, there is disclosed a video-equipped endoscope system employing a solid state image sensor. Color separation is created by means of a filter wheel that is adapted to rotate red, green and blue filters through the beam of a single arc lamp used to illuminate the target. The recorded images are processed in sequence and laid down one over the other upon a Braun tube. Beyond the disadvantages relating to sequentially laying down color-separated images upon a Braun tube, the Kakinuma et al system makes no provision for proportionally balancing the color separated images. Consequently, when the images are brought together to form a video picture, the picture may not faithfully reflect the original colors found in the target region. The Kakinuma et al endoscope, therefore, has little use as a medical diagnostic instrument.
In a co-pending U.S. application Ser. No. 487,070 filed in the name of Sarofeen et al, there is disclosed a color-balancing system for use in a video-equipped endoscope system utilizing a filter wheel to create color-separated images. In this particular system, an arc lamp is employed to illuminate the target region. The amount of charge placed upon the discharge capacitor of the lamp is closely controlled in order to limit the amount of illumination received by the target during each color-imaging sequence. The amount of illumination utilized during each color-imaging sequence is manually preset to attain a proper balance between the colors used to create the video picture. Once preset, the system will automatically maintain the three-color illumination levels constant until such time as the levels are readjusted.
The viewing head of the video endoscope, particularly when used in a medical application, is required to operate within a very confined region containing little, if any, ambient light. The light used to illuminate the target is generally brought into the target region from a remote source via fiber bundles. However, as the head is moved towards or away from the target, the amount of reflected light recorded by the CCD image sensor changes dramatically. Although an automatic gain control can be included in the video system, this in itself may not be sufficient to prevent the picture from blooming when the CCD image sensor reaches a full well condition. As will be explained in greater detail below, the present invention is designed to act in conjunction with an automatic gain control circuit to reduce the output of the illumination lamp without disturbing the proportional color balance of the color separated images.